This invention relates to fluoropolymers and more particularly to curable polyfluoro prepolymers.
Fluoropolymers are used as binders in high-density explosives and propellants and in energetic compositions requiring a high degree of thermal stability. For example, fluoropolymers are used extensively in such compositions which are pressed. However, very few fluoropolymers exist which have functional groups such as hydroxy which are suitable for curing and thus can be used as binders in castable or extrudable compositions. Two known examples of such polymers are the fluoropolyethers FC2202 and L9019 made by the 3M Company. While useful for some purposes, these polymers are expensive and completely fluorinated. The absence of hydrogen limits the compatibility of these polymers with conventional plasticizers, curing agents, and other polymers.
Horst G. Adolph and Judah M. Goldwasser in U.S. Pat. No. 4,740,628, entitled "2,4,4,5,5,6,6-Heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-dio l Polyformal and Method of Preparation," disclose a polyfluoro prepolymer which is miscible with conventional plasticizers, curing agents, and other polymers. They disclose that 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal is useful as a block for preparing block-copolymers with non-fluorinated polymers such as polyethylene glycol or poly(4,4-dinitroheptane-1,7-diol). Moreover, 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal may be cured by conventional means such as commercially available polyisocyanates.
Adolph et al. disclose the preparation of 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal by the reaction of 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol with formaldehyde and concentrated sulfuric acid in the presence of dichloromethane. The crude product is purified by treatment with hydrogen peroxide and by heating under vacuum to remove volatile components. This procedure worked satisfactorily on a small laboratory scale. However, when this procedure was used repeatedly and on a larger scale, it was found that the molecular weight was not reproducible and that difficulties in separation of the organic and aqueous layers were frequently encountered. A more reliable and facile process was therefore needed to permit preparation of the 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal on a larger scale.